There are many types of vascular defects that can be treated by blocking the defect. One example of such a defect is an aneurysm, which is a permanent, abnormal blood-filled dilatation or ballooning of a blood vessel that may be congenital or the result of disease. Aneurysms typically have thin walls vulnerable to rupture. If an aneurysm ruptures, the resulting hemorrhage that can put injurious pressure on surrounding tissue, impair downstream blood flow, and even cause death. Another example of a vascular defect is an atriovenous malformation--a typically congenital shunt formed between an artery and a vein that often carries a substantial blood flow. One of the principal complications in treating these and other vascular defects is the blood flow in the adjacent vessels which impairs treatment, but should be maintained for the health of the patient.
Current treatments for aneurysms include ernbolizing the aneurysm to remove the dilatation or balloon from the wall of the vessel. In the most mature technique, the surgeon accesses the region of the aneurysm under direct visualization and places one or more aneurysm clips on the opening or "neck" of the aneurysm. While this conventional surgical technique has a high rate of success, it is highly invasive and for that reason it is undesirable. More recently, less invasive techniques have been developed for the treatment of aneurysms. One such technique involves the introduction of small wire coils into the aneurysm. A catheter is navigated to the site of the aneurysm, and the coils are delivered through the lumen of the catheter into the aneurysm. The coils reduce the blood flow through the aneurysm, which results in clotting within the aneurysm. This coiling procedure can be time consuming both in navigating the catheter through the vasculature to the site of the aneurysm, and in introducing the coils into the aneurysm. In some cases, the shape of the aneurysm allows the coils to escape from the aneurysm, requiring the coil to be retrieved and replaced.
Another less invasive technique for treating vascular defects is the delivery of embolic materials to the site of the vascular defect to occlude the defect. In the case of an aneurysm a balloon is inflated over the neck of the aneurysm and a liquid embolic agent is introduced into the aneurysm. Attempts have been made to deliver embolic agents directly into the dilation or balloon of the aneurysm. Embolic agents have also been used to occlude atriovenous malformations, but it can be difficult to accurately deliver the embolic agents. In one of the more common procedures a catheter is navigated to the site of the atriovenous malformation and particles of polyvinyl alcohol with sizes selected for the particular application are introduced. This procedure requires guessing at the proper size of the particles and there is limited control over the placement of the particles, which upon release follow the path of greatest flow.
Alksne, "Iron-acrylic Compound for Stereotactic Aneurysm Thrombosis." J. Neurosurg. 47:137-141 (1977), incorporated herein by references, discloses injecting an iron-acrylic mixture into the dome of an aneurysm, and holding the mixture in place with a magnet inside the body. Gaston et al., "External Magnetic Guidance of Endovascular Catheters with Superconducting Magnet: Preliminary Trials" J. Neuroradiol. 15: 137-147 (1988), incorporated herein by reference, discloses delivering magnetic particles with an external source magnet. Evans, U.S. Pat. No. 5,702,361 "Method of Embolizing blood Vessels" incorporated herein by reference, discloses various embolizing agents including polymers and/or adhesives. Granov et al., U.S. Pat. No. 5,236,410, "Tumor Treatment Method," incorporated herein by reference, discloses the use of magnetic materials in tumor treatment.
Difficulties with prior embolic agents include complications from the delivery method, which sometimes employed balloons to temporarily block flow through the vessel and the difficulty in controlling and containing the embolic agents, which allows some material to escape and block downstream vessels.
In addition, some embolic agents did not adequately adhere to the vessel walls, allowing blood to seep between the embolic plug and the vessel wall. When biocompatible adhesives were used, the adhesives tended to adhere to the delivery equipment, resulting in a potentially fatal attachment of the delivery catheter to the embolic plug, or the pulling of a "string" of embolic material from the body of embolic material as the delivery catheter was retracted.
Another limitation on the use of embolic agents has been the limited ability to simultaneously view the ejection of the embolic agent under fluoroscopy of adequate quality. Conventional image intensifiers cannot operate in the presence of magnetic fields much larger than the relatively weak field of the earth (about 0.5 gauss). Fields of hundreds to thousands of gauss are required to control magnetic embolic agents, and these fields must be projected at distances large enough to reach aneurysms inside the body. External magnets whicji project such strong fields prohibit the use of conventional image intensifiers near the patient. One attempted solution is to use mirrors to project the X-ray image impinging on a phosphor plate to a remote camera, but this approach is not practical for human operating room procedures. First, the loss of light intensity due to the optical converter would require increased X-ray intensity which is unacceptable in clinical hospital settings. Second, the dim light being projected would require that the optical path to the distant camera be entirely black. This is difficult to implement with moving imaging systems.
Despite these and other possible difficulties, flowable embolic agents offer advantages over objects including the ability to uniformly fill the defect, and the relative ease of delivering a flowable embolic agent versus multiple discrete objects, such as coils.